I have this as middleware, which is basically the same code as the official NextJS website:

export function middleware(request:NextRequest){
    //do something

    return NextResponse.next()
}

export const config = {
    matcher: ["/((?!api|_next/static|_next/image|favicon.ico|.*\.svg).*)", "/((?!register).*)"]

}

This would be matching every route that it would access, I only wanted the middleware to access anything BUT a route called register, so I removed the .* from both regexp and it worked!

export const config = {
    matcher: ["/((?!api|_next/static|_next/image|favicon.ico|.*\.svg))", "/((?!register))"]

}

I'm leaving this here, just in case someone needs it or to see if my approach was incorrect.